Recently, a need exists to promote planned performance of steps against global warming. Of the total greenhouse effect gases discharged in Japan, energy-derived CO2 emissions account for about 90% in recent years. Moreover, of the total power generated, coal-fired power generation discharges 50% CO2. Thus, coal-fired power generating facilities are required to promote the use of new types of energy having low environmental impact.
Against this background, the “Special Measures Law Concerning the Use of New Energy by Electric Utilities” (hereinafter, the “RPS (Renewables Portfolio Standard) Law”) was enacted in Japan. The RPS Law is intended to promote the use of new energy by annually imposing an obligation on electricity retailers to use a predetermined amount or more of electricity derived from new energy or the like, according to the amount of electricity the retailers sell.
The RPS Law permits the electricity retailers to build a new coal-fired power generating facility only if the coal-fired power generating facility is a biomass-mixed system that burns biomass as auxiliary fuel. Existing facilities are also required to introduce the biomass-mixed combustion system.
Organic substances repeat a cycle of decomposition, absorption, and release on Earth. Equilibrium can thus be achieved for the amount of CO2 discharged by biomass energy by having a source of absorbing the equal amount of CO2. Biomass power generation that uses woody biomass fuel as a circulative resource therefore does not practically increase CO2 load in the atmosphere and thus carries the weight of expectations as new energy. Easily collectable woody biomass includes wood pellets and wood chips.
Additionally, use of the biomass fuel as auxiliary fuel in the coal-fired boiler not only saves fossil fuels and reduces the amount of CO2 emissions, but also achieves reduction in NOx contained in combustion exhaust gases because the biomass fuel contains low nitrogen content.
As a combination type coal-fired boiler that has hitherto been used, a known biomass-mixed fired boiler includes a conventional pulverized coal burner or a biomass-mixed combustion burner that supplies coal and biomass fuel simultaneously to thereby burn pulverized fuel that is a mixture of pulverized coal and biomass fuel. A typical system uses a conventional pulverized coal-fired boiler and manufactures a mixed fuel of pulverized coal and biomass by, for example, adding a woody biomass material to a roller or other type of mill that crushes coal into fine powder. The system then conveys the mixed fuel on conveyance air and burns the mixed fuel using the pulverized coal burner.
The roller mill pulverizes coal into fine particles of commonly 200 μm or less, preferably about 70 μm, in order to improve combustion efficiency of the burner. At this time, the biomass fuel is also pulverized into fine particles. If, in contrast, the coal and the woody biomass are loaded and processed simultaneously in the roller mill, an aggravated product grain size results with a resultant increase in the amount of coarse components of 100 μm or more. FIG. 7 compares a pulverized grain size distribution when 5% woody biomass is mixedly processed in the roller mill with that when only coal is processed. In the graph showing grain sizes of product fuels in FIG. 7, the abscissa represents sieve mesh in a logarithmic scale and the ordinate represents weight percentage of fuel that has passed through the sieve mesh. The graph reveals that mixing the woody biomass expands the grain size distribution of the product fuel both in coarse and fine directions.
In addition, the woody biomass fuel and the pulverized coal have combustion characteristics different from each other. For example, the woody biomass has a volatile content twice as high as that of coal. The wood pellet has a calorific value of ⅔ of that of coal and the wood chip has a calorific value of ½ of that of coal. The wood pellet and the wood chip have an ash content of 1/10 or less of that of coal. Thus, the mixing ratio is limited in co-firing the biomass fuel with coal in a burner designed as a pulverized coal burner.
The biomass fuel mixing ratio in the pulverized coal fired boiler is 3% in terms of actual industrial applications and the limit is estimated to be about 5%.
With future trends taken into consideration, if a mixed fuel burning ratio of about 30% by weight can be achieved in the biomass-mixed fired boiler that co-fires pulverized coal to which the woody biomass fuel is added as auxiliary fuel, possibility of utilization of the biomass-mixed fired boiler is expected to greatly increase. A high mixed fuel burning ratio of the biomass fuel cannot be obtained from using the pulverized coal burner and thus conceived is the introduction of a biomass combustion burner.
The finer the woody biomass is pulverized, the more the power is required in pulverization, which increases the unit requirement. On the other hand, the woody biomass fuel is easier to burn than coal if particle diameters are the same, which eliminates the need for making small the pulverized grain size. The woody biomass fuel and the pulverized coal have combustion characteristics different from each other. Thus, to burn the woody biomass fuel efficiently, ideally, the biomass combustion burner specifically designed for use with the woody biomass fuel is used.
To use a biomass combustion burner, a pulverizing mill is operated under conditions suitable for the woody biomass fuel independently of the pulverized coal. The biomass-mixed fired boiler can be operated with a suitable mixed fuel burning ratio selected as against the coal used in the pulverized coal burner.
The mixed fuel burning ratio for the boiler is determined according to the numbers of pulverized coal-fired burners and biomass combustion burners and the combustion efficiency.
Patent Document 1 discloses a biomass combustion burner that is applied to a biomass-mixed fired boiler that loads pulverized coal and woody biomass fuel through respective lines into a furnace for combustion. The disclosed biomass combustion burner includes a biomass fuel jet nozzle. The biomass fuel jet nozzle includes a disperser at a center thereof, the disperser preventing uneven flow of the biomass fuel, and a venturi disposed upstream inside the nozzle, the venturi increasing flow velocity of the fuel to thereby cause biomass fuel particles to collide with the disperser. The biomass fuel jet nozzle further includes a flame stabilizer disposed at a leading end thereof, the flame stabilizer having a stepped enlarging structure for sharply expanding the biomass fuel stream. The biomass combustion burner further includes a combustion air nozzle on the outside of the biomass fuel jet nozzle, the combustion air nozzle supplying a secondary air swirl flow.
The biomass combustion burner is optimized for burning a predetermined amount of biomass fuel. The number of biomass combustion burners to be installed may be determined according to the amount of biomass fuel to be processed required in the furnace to which the burners are applied. The arrangement disclosed in patent document 1 has a mixed fuel burning ratio of 15%.
It is noted that, preferably, the biomass combustion burner is disposed between a pulverized coal combustion burner and a two-stage combustion air jetting port.
Patent Document 2 discloses a boiler that includes a biomass-mixed combustion burner burning pulverized coal and biomass fuel and a boiler that includes a starting or auxiliary burner that functions also as a biomass fuel burning burner that burns biomass fuel supplied intermittently thereto. Patent Document 2 does not, however, describe any specific configuration of the biomass combustion burner, problems encountered during its use, solving means, and the like.
Patent Document 3 discloses a pulverized coal combustion burner. The disclosed burner is adapted to pulverized coal that has a greater calorific value, a greater amount of air required for combustion, and greater specific gravity than those of the biomass fuel and thus has a small optimum grain size. To burn the woody biomass fuel with high efficiency, therefore, the burner is required to be optimized to suit the woody biomass fuel.